When a liquid hose containing an array of hydrophones arrays is towed longitudinally through water there is created in the wall of the hose traveling or proprogating waves or undulations known as breathing waves. These waves are essentially minute axisymmetric radial deformations in the hose wall which propagate therealong progressively past each transducer in turn and interfere with transducer output. These breathing waves are generated by a number of conditions including excitation imposed by the turbuluent boundary layer, and their numbers and frequencies are related to tow speeds as well as to the geometry and condition of the array.
These breathing waves act to pulse-pressurize the liquid filling which surrounds each hydrophone. It is desirable that the hydrophones provide an output signal responsive to a distant exterior sound source without interference (noise) from local hydrostatic pressure pulse such as are caused by the breathing waves. The signal to noise ratio of an array hydrophone can therefore be increased by measuring the circumferential strain in the hose wall and subtracting a mulitple of the resulting signal from the total hydrophone output, thereby leaving a signal responsive only to distant sound or acoustic waves.
Cable strumming has long been recognized as a noise problem for towed hydrophones. It is a source of noise developed by transverse vibration of a cable upon being drawn through water, and it is a source of serious noise interference with acoustic input into a hydrophone. The cable's transverse vibrations cause axial accelerations decelerations at its free end where it is attached to a hydrophone housing. This results in a pulsating axial motion introduced into the housing which in turn is transferred to the hydrophone. U.S. Pat. No. 3,860,899 (Watlington) recognized the cable-strum axial vibration problem and provides an arrangement for generating an electric signal proportional to the axial accelerations and decelerations which is introduced in opposition to the electric signals generated by the hydrophones. The Watlington arrangement employes a velocity device which is connected to to the tow cable to insure that it moves in axial unison (forward and backward) wit the cable strumming. The present invention is directed to a different problem, that of traveling breathing waves. The problem is not recognized or treated in Watlington.